In establishments where heat of relatively a lower temperature such as waste heat is generated, for example, in a cement kiln, an iron making factory, a metal refinery, a chemical factory, an incinerator, a geothermal power plant and so on, there is provided a steam turbine power generation system that generates power by driving a steam turbine with a steam, which is recovered or generated by a waste-heat boiler or the like from heat such as an exhaust gas or the like, in order that an energy resource can be effectively used.
Patent Document 1 describes a power generation system that utilizes waste heat of a cement calcination plant.
In the cement calcination plant, for example, a large amount of exhaust gas of 300° C. to 400° C. is discharged from a suspension preheater (hereinafter referred to as just a preheater or PH), and a large amount of exhaust gas of about 250° C. to 300° C. is discharged from an air quenching cooler (hereinafter referred to as just a quenching color or AQC). The exhaust gas from the PH and the exhaust gas from the AQC largely differ in temperature and waste heat quantity, depending on an operation condition of the plant. In particular, a temperature of the exhaust gas from the AQC largely varies in a repeatedly cyclic manner.
In a conventional power generation system that recovers a waste heat of a cement calcination plant, there are provided a PH boiler using an exhaust gas of a PH as a heat medium and an AQC boiler using an exhaust gas of an AQC as a heat medium, for example. High-pressure steam, which is generated by both boilers, is introduced into a high-pressure stage of a steam turbine, and low-pressure steam, which is generated by separating, with a flasher, surplus hot water obtained in a heater unit of the AQC boiler into steam and liquid, is introduced into a low-pressure stage, so as to drive the steam turbine. An exit-gas temperature of the heater unit of the AQC boiler is about 100° C., and an exit-gas temperature of the PH boiler is 200° C. to 250° C.
However, in the cement calcination plant after the AQC boiler has been installed, although there are further heat sources of a lower temperature such as an exhaust gas from a lower-temperature part of the AQC and an exhaust gas whose waste heat has been recovered, it is difficult to generate steam capable of being supplied to a suitable stage of the steam turbine, by low-temperature media from these low-temperature heat sources. Thus, the heat of these exhaust gases are not recovered.
In a sintering cooler in an iron making factory, there is also utilized a power generation system that recovers waste heat of a high-temperature exhaust gas of 250° C. to 450° C. However, waste heat of a lower-temperature exhaust gas of the sintering cooler is not recovered.
As described above, conventionally, since it is difficult to generate steam capable of being supplied to a steam turbine, from a low-temperature heat source of, e.g., not more than 150° C. in a factory, waste heat thereof is not recovered.
However, in recent years, in view of an environmental problem, a cost saving request and, in particular, a request of selecting a safe power generation means based on the Tohoku earthquake, further improved waste heat utilization is desired. Namely, it is now desired that heat of an exhaust gas having a further lower temperature, which is generated in a factory or the like, can be efficiently used.
As a method of obtaining power by recovering heat from a low-temperature heat source, there is a binary cycle power generation system disclosed in Patent Document 2, for example. The binary cycle power generation system is a closed turbine power generation system of a Rankine cycle type which drives a steam turbine to generate power in the following manner. Namely, a medium whose boiling point is lower than that of water such as pentane or trifluoroethanol is used as a working medium. With the use of steam obtained by evaporating the medium with heat from a low-temperature heat source, the steam turbine is driven. Such a binary cycle power generation system can recover heat from steam of a low temperature and hot water of a low temperature, which have not been conventionally utilized.
However, the binary cycle power generation system is a power generation system suited for a low-temperature heat source. Thus, when there is also a high-temperature heat source, a steam turbine for binary cycle which uses an expensive heat medium should be added, in addition to a highly efficient steam turbine system using high-temperature steam. Therefore, the binary cycle power generation system is unsatisfactory in terms of cost effectiveness.
Patent Document 1: JP2008-157183A
Patent Document 2: JP2004-353571A